Hydraulic valves, in particular manifold valves, such as 3/2 directional control valves, are used for example in hydraulic systems of vehicle transmissions as pilot control elements. The valves are respectively inserted into a so-called valve cartridge, which in turn is inserted into a valve block. The valve cartridge has through openings for the passage of fluid, such as a hydraulic oil, flowing to the valve. The through openings are respectively assigned to a particular port of the valve to be inserted into the valve cartridge.
The hydraulic fluid of a hydraulic system of a vehicle transmission may have impurities. For this reason, in the through openings of the valve cartridge, through which the fluid flows into the valve, there is respectively arranged a filter device, for example a strainer, to filter out the impurities, so that these do not get into the valve. A valve cartridge usually consists of an injection-molded plastic part, the strainers being directly molded therein too upon the manufacturing of the valve cartridge by injection-molding. In so doing, the strainers are injection-molded onto the inside diameter of the valve cartridge, because a different arrangement of the strainers would not be possible or only with high effort due to the boundary conditions preset by the injection molds and tools.
The opening cross-section of each of these through openings, however, is diminished by the filter device, so that an undesirable pressure drop occurs. This is particularly critical in particular in the low-temperature range, for example in a range of −40° C. to 0° C., here the hydraulic medium becomes viscous due to the low temperatures. This increases the reaction time in the hydraulic system of the vehicle transmission.
As to avoid this problem or at least reducing it, it is desirable to enlarge the opening cross-sections of the through openings and in particular also those of the strainers. However, an enlargement of the strainer areas at the inside diameter of the valve cartridge is constructively restricted, because the inner construction of the valve cartridge is determined by the valve to be accommodated. A different arrangement of the strainers, namely for example at the greater outside diameter of the valve cartridge, however, cannot be realized by injection molding method, as already mentioned. Other methods for attaching larger strainers, such as for example an adhesive bonding onto the outside circumference of the valve cartridge or also a plugging or snapping of separate strainers onto the valve cartridge are possible, but cause additional effort and increased costs upon the manufacturing of the valve cartridge or are not optimal due to other reasons, for example because of a greater overall volume or lacking stability.